Electrospray ionization (ESI) sources that are coupled to on funnels often use an inline capillary (e.g., single or multiple) to introduce ions into the mass spectrometer (MS). FIG. 1 shows a conventional inline approach, in which a heated capillary is used to introduce ions from the ESI source directly into the ion funnel. The ion funnel then efficiently introduces ions into the mass spectrometer. However, when an inline capillary is used to introduce ions into the ion funnel, any incompletely desolvated liquid droplets generated by the ESI are inadvertently entered into the ion funnel and subsequently carried into the mass spectrometer due to the pressure gradient. In line approaches can thus cause contamination of downstream components of mass spectrometers. Contamination is greatest when the capillary, the ion funnel, the mass spectrometer, and other mass spectrometer elements are inline. This problem is more pronounced with multiple inlet capillaries used to increase the analyte signal, as multiple inlet capillaries significantly increase the quantity of ions introduced into the mass spectrometer, e.g., by as much as five-fold compared to single inlet capillaries with the same internal diameter (I.D.). The introduction of large volumes of gas can lead to rapid contamination of downstream mass spectrometer elements, thereby resulting in unstable signals, signal loss, and eventual complete loss of signal. One approach to mitigate contamination of downstream mass spectrometer elements is to place a jet disrupter into the ion funnel. However, the jet-disrupter also becomes contaminated. And, since the jet disruptor does not completely block liquid droplets and neutrals going into the mass spectrometer, this configuration still leads to contamination of mass spectrometer elements and to signal deterioration over time.
FIG. 2 shows an ion injection approach known in the art that incorporates an inlet capillary placed between a repeller plate and a first electrode orthogonal to the entrance of the ion funnel. In this configuration, the repeller plate is parallel to the first electrode of the ion funnel at a distance of approximately 12 mm. Both the repeller plate and first electrode are energized with DC only potentials. A strong electric field between the repeller and the entrance to the ion funnel diverts ions into the ion funnel. However, when a multiple inlet capillary, or a larger (e.g., 1 mm I.D.) single inlet capillary is used, this ion injection approach does not perform as expected. Evaluation shows the signal intensity reaches a lower threshold compared with a single inlet capillary of the same I.D., as the DC field between the repeller electrode and the first funnel electrode is insufficient to oppose drag forces resulting from the greater gas loads generated by the multiple inlet capillary. Therefore, the DC field does not properly divert ions into the ion funnel at increased gas loads. And, practical limitations such as electrical discharge occur at higher electric fields which also limits DC fields that can be placed between the repeller electrode and the first ion funnel electrode. Accordingly, new inlet designs are needed that permit higher gas loads but do not increase the risk of contamination of downstream elements.